Compliant foil-fluid bearing support arrangement

ABSTRACT

A foil-fluid bearing assembly adapted for supporting a shaft within an outer bearing housing. The foil-fluid bearing assembly comprises a bearing sleeve having an outer surface and an inner circumferential surface diametrically sized to receive the shaft therewithin such that an annular clearance gap, radially defined between an outer surface of the shaft and the inner circumferential surface, is adapted for accommodating fluid therein and axially extends along a substantial portion of a length of the bearing sleeve, and has a foil element disposed therewithin. Compliant bearing supports between the outer surface of the bearing sleeve and the outer bearing housing, comprises at least two independent compliant support elements respectively having a first and a second modulus of elasticity, wherein the first modulus of elasticity is less than the second modulus of elasticity. Angular misalignment capability for the foil-fluid bearing is provided by the compliant support elements.

TECHNICAL FIELD

[0001] The present invention relates generally to a foil-fluid bearing,and in particular, to a foil-fluid bearing comprising compliantsupports.

BACKGROUND OF THE INVENTION

[0002] Foil-air bearings are advantageous as they operate efficientlyover a large temperature range, have increasing load capacity asrotational speeds increase, can handle severe environmental conditions,and are more reliable than rolling element bearings because they requirefewer parts to support the rotating assemblies and do not needlubrication. However, they also require inherently tight tolerancecontrols on the bearings themselves and their housings in order tomaintain a very close alignment between each radial bearing in a rotorsystem in order to operate effectively. Bearing misalignment can becaused by excessive manufacturing tolerances, operational loads orthermal distortion of the bearing housing, and can result in shaftangular misalignment causing unacceptable whirl instability. Existingfoil-air bearings also require a good control of both radial stiffnessand damping characteristics of the inner foil support structure.

[0003] The use of hydrodynamic fluid journal bearings having compliantfoils is known. The inner foils of the bearing are arranged such thatthe end sections of the foils themselves are more compliant than thecentral section of the foils, and can deflect independently in order toprovide improved conformity to a misaligned rotor supported by thebearing. Strong misalignment forces can thereby be tolerated. U.S. Pat.No. 4,274,683 teaches such a compliant foil bearing. However, thesehydrodynamic journal bearings require metallic corrugated foilassemblies that are comprised of several separate sections of foils,which must each be independently flexible and able to deflect differentcontrolled amounts. Introducing independent flexibility to the metallicfoils themselves considerably adds to the complexity, and consequentlycost, of the bearing. Additionally, as each bearing requires severaldiscrete metallic foil sections having different flexibility, thecomplexity and manufacturing costs are further increased.

[0004] There remains a need to provide a foil-fluid bearing capable oftolerating angular misalignment, without significantly adding complexityto the internal construction of the bearing, and consequently withoutsignificantly adding to material and manufacturing costs.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide an improvedfoil-fluid bearing capable of increased angular misalignment tolerance.

[0006] It is an object of the present invention to provide a foil-fluidbearing having a simplified compliant support arrangement.

[0007] Therefore, in accordance with the present invention, there isprovided a foil-fluid bearing assembly adapted for supporting a shaftwithin an outer bearing housing, the foil-fluid bearing assemblycomprising: a bearing sleeve having an outer surface and an innercircumferential surface diametrically sized to receive the shafttherewithin such that an annular clearance gap is radially definedbetween an outer surface of the shaft and the inner circumferentialsurface, the annular clearance gap adapted for accommodating fluidtherein and axially extending along a substantial portion of a length ofthe bearing sleeve; a foil element disposed within the annular clearancegap; a compliant bearing support contiguously disposed between the outersurface of the bearing sleeve and the outer bearing housing, andpermitting elastic deflection of the compliant bearing support,independent of movement of the foil element; and the compliant bearingsupport comprising at least two independent compliant support elementsrespectively having a first and a second modulus of elasticity, whereinthe first modulus of elasticity is less than the second modulus ofelasticity; whereby angular misalignment capability for the foil-fluidbearing is provided by the compliant bearing support.

[0008] There is also provided, in accordance with the present invention,a foil-fluid bearing assembly adapted for rotatably supporting a shaftwithin a bearing housing, the foil-fluid bearing assembly comprising: abearing sleeve having an outer surface and an inner surface defining aninner race, said inner race diametrically sized to receive a rotatingshaft therewithin and maintain an annular clearance gap radially betweensaid rotating shaft and said inner circumferential surface, the annularclearance gap axially extending along a substantial portion of a lengthof the bearing sleeve, the bearing sleeve adapted, in use, to supply asupport fluid to substantially fill said annular clearance gap; a foilelement circumferentially disposed between said inner race and saidshaft; a compliant bearing support disposed between the outer surface ofthe bearing sleeve and the bearing housing; and the compliant bearingsupport having at least two independent compliant support elements, saidat least two independent compliant support elements respectively havinga first and a second modulus of elasticity, wherein the first modulus ofelasticity is less than the second modulus of elasticity.

[0009] There is additionally provided, in accordance with the presentinvention, a foil-fluid bearing assembly comprising: a compliantlysupported bearing sleeve, radially disposed between a rotating shaft andan outer housing, such that an annular clearance gap, adapted forreceiving a support fluid therein, is provided between the rotatingshaft and the bearing sleeve; a foil element being radially disposedwithin the annular clearance gap; the bearing sleeve being compliantlysupported within the outer housing by at least two independent compliantsupports, radially disposed between the bearing sleeve and the outerhousing; and the two independent compliant supports respectively havinga first and second modulus of elasticity, the first modulus ofelasticity being less than the second modulus of elasticity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Further features and advantages of the present invention willbecome apparent from the following detailed description, taken incombination with the appended drawings, in which:

[0011]FIG. 1 is an axial cross-sectional view of a shaft supported ateither end by a compliantly supported foil-fluid bearing according tothe present invention.

[0012]FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1.

[0013]FIG. 3 is a partial cross-sectional view enlarged from region 3 ofFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] Referring first to FIG. 1, each foil-fluid bearing assembly 10 ofthe present invention generally comprises a bearing sleeve 16 supportinga rotating shaft 12 within an outer bearing housing 14. At least onecompliant support 30, 32 is radially disposed between the bearing sleeveand the outer housing, and is capable of elastically deflecting totolerate misalignment of the bearing sleeve with respect to the outerhousing. The bearing sleeve 16 comprises an inner bore therethroughhaving a diameter such that a precise annular radial gap 24 is definedbetween an inner circumferential surface of the sleeve and an outersurface of the shaft. Much like all foil bearings, the present bearingcomprises an inner foil member 28, disposed within the annular radialgap 24. The foil member 28 is preferably comprised of at least twofoils, an inner smooth foil providing a fluid film running surface andat least one outer corrugated foil or equivalent spring foil. Manycorrugated foils can be radially stacked between the inner smooth foiland the inside circumferential surface 18 of the bearing sleeve 16. Thecorrugated foils provide some of the low stiffness and damping requiredfor the compliance of the smooth foil. The smooth foil provides a smoothrunning surface for the film of fluid that forms within the annularradial gap 24.

[0015] Traditional air bearings having sleeves rigidly fixed to thesurrounding housing, require very tight tolerance controls on thebearings and their housings, in order to maintain a good alignmentbetween each of the radial bearings of a rotor system.

[0016] The present foil-fluid bearing preferably comprises threecompliant supports installed on each bearing sleeve, each compliantsupport contiguous with, and radially disposed between, the bearingsleeve and the outer bearing housing. A central compliant support,axially located between two compliant end supports, having a highermodulus of elasticity than the two end compliant supports, permits thebearing sleeve to swivel around the stiffer middle compliant support.

[0017] This permits the compliant bearing to self-align with anotherradial bearing supporting the shaft or rotor system. Such a shaft orrotor system could for example, have at least another similar compliantfoil-fluid bearing at its opposite end, as shown in FIG. 2. Similarly,the present compliant foil-fluid bearing could be used on a shaft orrotor system in combination with any number of other bearings. Thisenables greater tolerance for bearing misalignment, which can resultfrom any one of, or combination of, several factors includingmanufacturing tolerances, operational loads, and thermal displacementsof the housing.

[0018] Referring now to FIG. 2 and FIG. 3, the foil-fluid bearingassembly 10 of the preferred embodiment comprises the bearing sleeve 16comprising an inner circumferential surface 18, and an outercircumferential surface 20 having three annular support points thereon.The support points each comprise an annular notch, disposed radiallyoutwardly and being adapted for receiving an annular compliant supporttherein. Specifically, the middle support point 22 has annular notch 25formed therein, and the two outer support points 23 define annularnotches 27 therein. The central support point notch 25 receives annularcompliant middle support 32, and the two outer support points notches 27receive annular compliant end supports 30. As mentioned above, the twocompliant end supports 30 have a predetermined lower modulus ofelasticity, and are therefore less stiff, than the middle compliantsupport 32. While the middle support point 22 is preferably at the axialmidpoint of the bearing sleeve 16, the distance between the middlesupport point and the end support points can be modified as required toprovide more or less rotational flexibility of the bearing sleeve aboutan axis perpendicular to the longitudinal axis 13 of both the bearingsleeve and the shaft 12. Both the stiffer central compliant support 32and the outer compliant supports 30 are in contacting engagement withthe inner circumferential surface 15 of the outer bearing housing 14.

[0019] The generally cylindrical bearing sleeve 16 comprises aconcentric central bore therethrough, defined by the innercircumferential surface 18 that is precisely diametrically sized toreceive the shaft therewithin such that a controlled annular radial gap24 is provided between the outer surface 26 of the shaft 12 and theinner circumferential surface 18 of the sleeve. The annular gap 24axially extends along the length of the bearing sleeve and is axiallyclosed off by inwardly projecting circumferential tabs 29 or mayalternately be axially retained by other mechanical features, dependingon the particular foil design. The corrugated foils 28 of the bearingare disposed within the annular gap 24, and extend about the fullcircumference of the bearing sleeve. The foils 28 of the present bearingpreferably have an axial length substantially equal to the length of thegap 24, however several separate sections of corrugated foils, eachextending about the full circumference of the sleeve but axiallyextending only along a fraction of the full axial length of the bearingsleeve, could nevertheless be used. The corrugated foils 28 aregenerally fixed at an outer radial edge to the inner circumferentialsurface 18 of the bearing sleeve, and extend freely into the annular gapat inner radial edges thereof.

[0020] The foils act much as is well known in foil bearings of the priorart. As opposed to traditional journal bearings wherein the radialclearance between the shaft and the bearing sleeve is fixed, the foilsof the foil-fluid bearing of this invention deflect radially outward inorder to provide a relatively constant radial fluid film clearance whenshaft growth due to temperature and centrifugal forces occurs. As theshaft turns, hydrodynamically generated pressure pushes the foils awayfrom the shaft, keeping the shaft completely fluid-borne. When the shaftradially grows due to thermal expansion, the foils get pushed radiallyfurther away, keeping the fluid film clearance relatively constant.

[0021] While foils can be themselves made flexible in order to providesome damping which is required to suppress whirl instability resultingfrom bearing misalignments, in order to be sufficiently compliant theprior art flexible foils must be considerably complex and are thereforeexpensive to manufacture. The present invention can use simple andrelatively inexpensive non-flexible foils, as damping to prevent whirlinstability and bearing misalignment is provided by the compliantbearing sleeve supports. As radial stiffness and the dampingcharacteristics of the bearing are provided by the compliant supports,externally from the inner foil construction of the bearing, significantmanufacturing cost reductions can be realised.

[0022] As the compliant supports can be removed and replaced with othercompliant elements having different stiffness, size or materialcharacteristics, the radial stiffness and misalignment tolerances of thepresent foil bearing can easily be modified as required for a specificapplication.

[0023] The compliant support arrangement of the present foil bearingpermits significantly improved capability to tolerate angularmisalignment between radial bearings. This subsequently permits the useof bearing housings that are less finely toleranced, resulting insignificant manufacturing cost savings.

[0024] A single central compliant support for the bearing sleeve couldbe used in specific limited applications, however care must be taken toensure that such applications, having the present foil-fluid bearingwith a single compliant support, are running at a rotational speedoutside the natural frequency envelope of the bearing and shaftassembly, otherwise excessive oscillation of the sleeve can occur. Thetwo compliant end supports located either side of the central, andstiffer, compliant support provide damping against such naturalfrequency oscillation of the sleeve. Therefore, the three pointcompliant support arrangement is preferable, however a single centralcompliant support for the bearing sleeve within the housing cannevertheless be used for applications that will always operate outsidethe above-mentioned rigid body mode frequency of the sleeve.

[0025] The total radial stiffness and damping characteristics of thepreferred embodiment of the present compliant support arrangement,wherein a stiffer central compliant bearing sleeve support is disposedbetween two compliant end supports having a lower modulus of elasticitythan the central support, can be controlled and varied as required byproper compliant material and size selection, as it would for two ormore compliant supports having equal stiffness. The rotationalflexibility of the bearings sleeve about an axis perpendicular to thelongitudinal axis of the shaft can also be varied as required, bychanging the distance between the stiffer middle compliant support andthe side compliant supports.

[0026] The foil-fluid bearing of the present invention is preferably anair bearing, although it can equivalently be used for foil bearingsusing a process fluid other than air, such as helium, xenon,refrigerants, liquid oxygen and liquid nitrogen.

[0027] The present foil-air bearing could be used in a variety ofapplications, however it is particularly intended for use in an aircycle machine, employed in an aircraft air conditioning system forexample. Almost all aircraft currently use air cycle machines in theirenvironmental control systems, and the present foil-air bearingparticularly lends itself for use in such an air cycle machine. Thepresent foil-air bearing is also particularly intended for use in a gasturbine engine. Such a foil-air bearing could, however, equally be usedin other rotating machinery applications, such as motor drivencompressors and other fuel driven turbomachines.

1. A foil-fluid bearing assembly adapted for supporting a shaft withinan outer bearing housing, the foil-fluid bearing assembly comprising: abearing sleeve having an outer surface and an inner circumferentialsurface diametrically sized to receive the shaft therewithin such thatan annular clearance gap is radially defined between an outer surface ofthe shaft and the inner circumferential surface, the annular clearancegap adapted for accommodating fluid therein and axially extending alonga substantial portion of a length of the bearing sleeve; a foil elementdisposed within the annular clearance gap; a compliant bearing supportcontiguously disposed between the outer surface of the bearing sleeveand the outer bearing housing, and permitting elastic deflection of thecompliant bearing support, independent of movement of the foil element;and the compliant bearing support comprising at least two independentcompliant support elements respectively having a first and a secondmodulus of elasticity, wherein the first modulus of elasticity is lessthan the second modulus of elasticity; whereby angular misalignmentcapability for the foil-fluid bearing is provided by the compliantbearing support.
 2. The foil-fluid bearing as defined in claim 1,wherein the independent compliant support elements include two endsupports, axially disposed at opposing ends of the bearing sleeve, and acentral support axially located between the end supports.
 3. Thefoil-fluid bearing as defined in claim 2, wherein the central supporthas the second modulus of elasticity and the two end supports both havethe first modulus of elasticity, thereby permitting the bearing sleeveto axially pivot about the central support.
 4. The foil-fluid bearing asdefined in claim 3, wherein the first modulus of elasticity is lowenough to permit deflection of the end supports, but high enough toprovide sufficient damping to prevent natural frequency oscillation ofthe bearing sleeve.
 5. The foil-fluid bearing as defined in claim 1,wherein the foil element substantially extends along the full length ofthe annular clearance gap.
 6. The foil-fluid bearing as defined in claim1, wherein the foil element comprises at least two foils.
 7. Thefoil-fluid bearing as defined in claim 6, wherein the foil elementcomprises a radially inner smooth foil and at least one radially outercorrugated foil.
 8. The foil-fluid bearing as defined in claim 1,wherein the foil-fluid bearing is an air bearing.
 9. The foil-fluidbearing as defined in claim 8, wherein the air bearing is adapted foruse in an air cycle machine.
 10. The foil-fluid bearing as defined inclaim 9, wherein the air cycle machine is adapted for use in an aircraftenvironmental control system.
 11. The foil-fluid bearing as defined inclaim 2, wherein the central support is substantially disposed at theaxial midpoint of the bearing sleeve.
 12. The foil-fluid bearing asdefined in claim 2, wherein the modulus of elasticity of eachindependent compliant support element is different.
 13. The foil-fluidbearing as defined in claim 2, wherein the end supports and the centralsupport are replaceable.
 14. The foil-fluid bearing as defined in claim2, wherein axial location of the compliant central support relative tothe compliant end supports can be modified to vary rotationalflexibility of the bearing sleeve about an axis substantiallyperpendicular to a longitudinal axis of the bearing sleeve.
 15. Afoil-fluid bearing assembly adapted for rotatably supporting a shaftwithin a bearing housing, the foil-fluid bearing assembly comprising: abearing sleeve having an outer surface and an inner surface defining aninner race, said inner race diametrically sized to receive a rotatingshaft therewithin and maintain an annular clearance gap radially betweensaid rotating shaft and said inner circumferential surface, the annularclearance gap axially extending along a substantial portion of a lengthof the bearing sleeve, the bearing sleeve adapted, in use, to supply asupport fluid to substantially fill said annular clearance gap; a foilelement circumferentially disposed between said inner race and saidshaft; a compliant bearing support disposed between the outer surface ofthe bearing sleeve and the bearing housing; and the compliant bearingsupport having at least two independent compliant support elements, saidat least two independent compliant support elements respectively havinga first and a second modulus of elasticity, wherein the first modulus ofelasticity is less than the second modulus of elasticity.
 16. Afoil-fluid bearing assembly comprising: a compliantly supported bearingsleeve, radially disposed between a rotating shaft and an outer housing,such that an annular clearance gap, adapted for receiving a supportfluid therein, is provided between the rotating shaft and the bearingsleeve; a foil element being radially disposed within the annularclearance gap; the bearing sleeve being compliantly supported within theouter housing by at least two independent compliant supports, radiallydisposed between the bearing sleeve and the outer housing; and the twoindependent compliant supports respectively having a first and secondmodulus of elasticity, the first modulus of elasticity being less thanthe second modulus of elasticity.
 17. The foil-fluid bearing assembly asdefined in one of claims 15 and 16, wherein the at least two independentcompliant supports include a central support having the second modulusof elasticity and opposing end supports having the first modulus ofelasticity, thereby permitting the bearing sleeve to axially pivot aboutthe central support.
 18. The foil-fluid bearing assembly as defined inone of claims 15 and 16, wherein the foil-fluid bearing is an airbearing.
 19. The foil-fluid bearing assembly as defined in claim 18,wherein the air bearing is adapted for use in one of an air cyclemachine and a gas turbine engine.
 20. The foil-fluid bearing assembly asdefined in claim 19, wherein the air bearing is adapted for use in anaircraft.